Magnetic resonance imaging (MRI) is widely used in the medical imaging field. When an object, e.g., a human body, is placed in a main magnetic field, the hydrogen atoms in the object may be polarized. A pulse of radio-frequency (RF) may excite hydrogen atoms in the object, causing the hydrogen atoms to resonate and absorb energy. When the RF pulse is removed, the hydrogen atoms may emit a RF signal with a certain frequency, and release at least part of the energy absorbed. A receiver placed outside the object may receive the emitted RF signal, based on which a magnetic resonance (MR) image may be produced.
MRI may produce images in, for example, the traverse plane, the sagittal plane, the coronal plane, or other planes essentially without an adverse impact on an object by exposing the object to radiation.
A magnet is a component in a magnetic resonance imaging system to produce a stable magnetostatic field. Superconducting magnets are widely used in MRI systems. The basic principle is to immerse one or more coils formed by a superconducting material in liquid helium at an extremely low temperature (about 4K), then to energize to coils to produce a magnetic field. The liquid helium and the coils may be held within a cryostat. Liquid helium is expensive and volatile, and so it is desirable to thermal isolate the interior from exterior ambient temperature condition to reduce boiling off helium.
FIG. 1A through FIG. 1D show a traditional cryostat, which has a multi-layer structure. FIG. 1A is a front view partly in section of a traditional cryostat, FIG. 1B illustrates the internal multiple layers structure and the connection of the traditional cryostat, FIG. 1C is a simplified schematic structure diagram of the traditional cryostat, and FIG. 1D is a side view of FIG. 1C. As shown in FIG. 1A through FIG. 1D, the cryostat includes an outer vessel 110 and an inner vessel 120. The inner vessel 120 may be configured to hold a cryogenic medium. One thermal shielding layer 130 may be employed between the inner vessel 120 and the outer vessel 110. The inner vessel 120 and the thermal shielding layer 130 are supported spaced apart from one another with multiple support elements within the outer vessel 110. The suspension support elements include multiple support elements 140 (represented by lines with black dot ends) placed radially and multiple support elements 150 (represented by lines with black triangle ends) placed axially. As shown in FIG. 1B, the support elements 150 placed axially may be a rod. The support elements 140 and the support elements 150 should be strong enough to withstand the gravity of the inner vessel 120, as well as the shock load during transportation, or a combination thereof. The shock load may be multiple times of the gravity of the inner vessel 120. Some extra preload may be applied to the support elements in advance to prevent tension losing at a low temperature. In this kind of cryostat embodiment the force of the suspension system is relatively simple and could be calibrated easily, but the number of support elements is large. Merely by way of example, there may be sixteen support elements, including eight support elements 140 placed radially and eight support elements 150 placed axially (as shown in FIG. 1C and FIG. 1D, because the traditional cryostat may be a symmetrical system, part of the support elements may overlap in the front view and the side view of the cryostat). The large number of support elements may result in a complicated system, a cumbersome assembly process, and high cost. Moreover, a large number of support elements may import more heat load into the inner vessel, which would degrade the stability of the thermal system and more cryogen loss.